The present invention relates to a current source.
Numerous current source configurations are known which are intended to provide a current level (referred to herein as the control current) according to a predetermined reference level. That reference level is in some circuits set by a separately supplied and accurately generated reference current Iref. One of the difficulties which can exist is that noise is injected into the reference current before it is used to control the value of the control current. Thus, errors arise in the value of the control current which attempts to match the noise affected reference current.
FIG. 1A serves to illustrate the problem of injected noise. A chip I.C. is illustrated with an external pin represented by node NA. An external resistor Rext attached to the pin is used to generate a very accurate reference current with high precision. On that pin, there can however exist a parasitic capacitor. If there is a ramping signal with high voltage at high frequency, which is sometimes the case in some switching power applications, currents Iinj can be injected at the node NA. That current will pass through the transistor labelled Q2 in FIG. 1A and affect the accuracy of the reference current.
It is an aim or the present invention to provide an improved current source which overcomes this defect.
According to the present invention there is provided a current source for generating a control current comprising: a reference current generator having a first output impedance and connected to supply a reference current to a circuit node; a control current generator having a second output impedance and connected to supply said control current to said circuit node, whereby an input voltage is generated at said circuit node based on the reference current, the control current and the first and second output impedances; a filter circuit connected to staid circuit node and arranged to filter said input voltage; and an amplifier connected to receive the filtered input voltage and arranged to control the level of the control current in dependence on the filtered input voltage.
In the described embodiment, the amplifier is a transconductance amplifier which comprises an NMOS transistor having its source connected to a resistive component and its gate connected to receive the filtered input voltage. Its drain is connected to the control current generator.
In the described embodiment the control current generator comprises a current source connected to a current mirror circuit which supplies the control current to said circuit node.
In the described embodiment the filter circuit comprises a resistive component connected between said circuit node and the amplifier input, and a capacitor connected between said resistor and a voltage supply terminal. With the polarities given in the following description, the capacitor is connected between one terminal of the resistor and ground. A further capacitor can be included within the filter connected between the other terminal of the resistor and ground.
For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings.